The present invention generally relates to measurement systems and methods and, more particularly, to a system and method for calibrating a scanning electron microscope.
In the semiconductor industry there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller features sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as the corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photo lithographic processes as well as high resolution inspection and measurement systems and/or methods. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which, for example, a silicon wafer is coated uniformly with a radiation-sensitive film (e.g., a photoresist), and an exposing source (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the film surface through an intervening master template (e.g., a mask or reticle) to generate a particular pattern. The exposed pattern on the photoresist film is then developed with a solvent called a developer which makes the exposed pattern either soluble or insoluble depending on the type of photoresist (i.e., positive or negative resist). The soluble portions of the resist are then removed, thus leaving a photoresist mask corresponding to the desired pattern on the silicon wafer for further processing.
In order to control quality in the design and manufacture of high density semiconductor devices, it is necessary to measure critical dimensions (CDs) associated therewith. Semiconductor device features having CDs of interest include, for example, the width of a patterned line, the distance between two lines or devices, and the size of a contact. CDs related to these and other features may be monitored during production and development in order to maintain proper device performance. As device density increases and device sizes decrease, the ability to carry out quick, inexpensive, reliable, accurate, high-resolution, non-destructive measurements of CDs in the semiconductor industry is crucial. The ability to accurately measure particular features of a semiconductor workpiece allows for adjustment of manufacturing processes and design modifications in order to produce better products, reduce defects, etc.
CDs are usually measured during or after lithography. Various operations performed during the lithography process may affect the critical dimensions of a semiconductor device. For example, variations in the thickness of the applied photoresist, lamp intensity during the exposure process, and developer concentration all result in variations of semiconductor line widths. In addition, line width variations may occur whenever a line is in the vicinity of a step (a sudden increase in topography). Such topography-related line width variations may be caused by various factors including differences in the energy transferred to the photoresist at different photoresist thicknesses, light scattering at the edges of the steps, and standing wave effects. Moreover, issues in connection with mask surface flatness, edge roughness of resist lines, charging of quartz material, chrome etch roughness, and phase shifts also factor into critical dimensions. Since these factors can greatly affect CDs, fast and reliable monitoring of semiconductor device features is important in order to guarantee acceptable device performance.
Different technologies are currently available to measure CDs associated with semiconductor devices. These include: optical microscopy, stylus profilometry, atomic force microscopy, scanning tunneling microscopy, and scanning electron microscopy. Scanning electron microscopes (SEMs) are commonly used for inspection and metrology in semiconductor manufacturing. The short wavelengths of scanning electron microscopes have several advantages over conventionally used optical microscopes. For example, scanning electron microscopes may achieve resolutions from about 100 to 200 Angstroms, while the resolution of optical microscopes is typically about 2,500 Angstroms. In addition, scanning electron microscopes provide depths of field several orders of magnitude greater than optical microscopes.
In a typical SEM wafer inspection system, a focused electron beam is scanned from point to point on a specimen surface in a rectangular raster pattern. Accelerating voltage, beam current and spot diameter may be optimized according to specific applications and specimen compositions. As the scanning electron beam contacts the surface of a specimen, secondary electrons are emitted from the specimen surface. Semiconductor inspection, analysis and metrology is performed by detecting these secondary electrons. A point by point visual representation of the specimen may be obtained on a CRT screen or other display device as the electron beam controllably scans a specimen.
Scanning electron microscopes (SEMs) operate by creating a beam of electrons accelerated to energies up to several thousand electron volts. The electron beam is focused to a small diameter and scanned across a CD or feature of interest in the scanned specimen. When the electron beam strikes the surface of the specimen, low energy secondary electrons (SEs) are emitted. The yield of secondary electrons depends on various factors including the work function of the material, the topography of the sample, the curvature of the surface, and the like. These electrons can be employed to distinguish between different materials on a specimen surface since different materials may have significantly different work functions.
Topographic features also affect the yield of electrons. Consequently, changes in height along a specimen surface may be measured using an SEM. Electron current resulting from the surface-emitted secondary electrons is detected and used to control the intensity of pixels on a monitor or other display device connected to the SEM. An image of the specimen may be created by synchronously scanning the electron beam and the display device.
Although SEMs can achieve resolution in the range of angstroms, calibration is difficult. For example, the magnification of an SEM may be calibrated by placing a sample of known dimensions, such as a chip or wafer having a conductor line of known width, in the instrument and measuring the line width of the sample. The magnification of the SEM is determined by dividing the SEM measurement of the image of the sample by the known dimension of the sample. The magnification calibration information may then be used to construct a calibration curve, or the SEM""s magnification controls may be trimmed accordingly.
Calibration according to these prior methods requires samples of known dimensions. Even if the actual dimensions of a sample are known, however, they may change. In particular, repeated usage of a single reference sample as a calibration standard results in degradation of the reference sample. Charge buildup on a reference sample caused by repeated measurement in an SEM affects the secondary electron emission. Contaminant deposition or buildup also has deleterious effects on measurement of a calibration standard reference sample over time. Conventional SEM calibration methods and systems do not account for degradation in reference features. Thus, repeated use of the same reference sample may lead to SEM calibration error over time, as the reference sample feature size changes.
The measurement of a calibration standard reference sample typically involves determining where an edge of the sample is. At the sub-micron range, an edge of a sample may be a complex waveform, as opposed to a flat line. Therefore, in measuring the sample, assumptions must be made as to edge location, which lead to errors. Where the calibration involves determining the length (or width) of a sample, two edges must be located, and thus the edge determination errors are doubled. Further, sample dimensions may vary as a function of temperature. The SEM electron beam may thus cause expansion of a reference sample after repeated use.
In order to reduce edge determination error, SEM calibration has also been done using a sample having a series of equally spaced lines. Such a sample could be a diffraction grating having a plurality of aligned parallel grooves. The SEM may be used to measure the pitch of the lines. While this method reduces some of the edge quantification errors associated with other SEM calibration methods, higher accuracy calibration methods are needed for SEMs used for measuring high density semiconductor devices.
Conventional SEM calibration methods and systems do not account for degradation of a calibration standard reference sample over time. For example, where a line width feature on a reference sample has a known width, repeated scanning of the feature by an SEM results in charge buildup. This reduces or hampers the ability of an SEM to obtain accurate measurements of the line width in the future. Corrosion deposition on a reference sample feature also prevents or hampers accurate readings. Mask references are particularly susceptible to carbon contamination. Because conventional SEM calibration methods and systems rely upon accurate SEM readings of a known reference feature dimension, inaccurate SEM readings of a calibration standard reference feature cause errors in measurements of workpiece features performed with the SEM.
The present invention provides a method and system for calibrating a scanning electron microscope, which minimizes or reduces the disadvantages associated with conventional methods and systems. In accordance with one aspect of the present invention, there is provided a method and system for calibrating an SEM using a reference having multiple features of different dimensions and/or spatial interrelationships, wherein more than one feature dimension or spacing is measured using the SEM prior to measuring a workpiece. The dimensional and/or spatial measurements from the reference sample are correlated to obtain one or more calibration factors for the SEM. The calibration factor or factors may then be correlated with a workpiece SEM measurement to obtain a workpiece critical dimension (CD). The method and system eliminate or minimize the effects of reference feature dimension variations, allowing such deviations to be detected and accounted for in the calibration of a SEM. In this regard, the correlation of the reference feature dimensions may comprise one or more of computing the slope of a curve, computing a zero offset, computing a calibration coefficient, curve fitting, stochastics, neural networks, artificial intelligence, data fusion techniques, and/or trending, according to another aspect of the invention.
According to another aspect of the present invention, one or more correlation curves may be generated from the reference sample SEM measurements for analysis in calibrating the SEM. The curves may comprise measured line width or pitch versus actual line width or pitch, or even actual or measured width versus actual or measured pitch. In this way, the correlation may utilize curve fitting, stochastics, neural networks, artificial intelligence, data fusion techniques, trending, and the like, to account for variations in reference sample feature dimensions in determining one or more calibration factors for the SEM.
In accordance with another aspect of the present invention, there is provided a method for calibrating a scanning electron microscope, comprising providing a reference sample having a first line with a first line width, and a second line with a second line width, and measuring the first and second line widths using the scanning electron microscope. The method further comprises correlating the first line width measurement with the second line width measurement to obtain at least one calibration factor.
In accordance with yet another aspect of the present invention, there is provided a method for calibrating a scanning electron microscope, comprising providing a reference sample having a first line set with generally parallel lines of a first pitch, and a second line set with generally parallel lines of a second pitch, measuring the first pitch using the scanning electron microscope, and measuring the second pitch using the scanning electron microscope. The first pitch measurement and the second pitch measurement are then correlated to obtain at least one calibration factor for the SEM.
In accordance with still another aspect of the invention, there is provided a method for calibrating a scanning electron microscope, comprising providing a reference sample having a first line set with generally parallel lines of a first line width and a first pitch, and a second line set with generally parallel lines of a second line width and a second pitch, measuring at least one of the first line width and the first pitch using the scanning electron microscope, and measuring at least one of the second line width and the second pitch using the scanning electron microscope. The method further comprises correlating at least one of the first line width measurement and the first pitch measurement with at least one of the second line width measurement and the second pitch measurement to obtain at least one calibration factor. The correlation may comprise one or more of computing the slope of a curve, computing a zero offset, computing a calibration coefficient, curve fitting, stochastics, neural networks, artificial intelligence, data fusion techniques, and trending, in order to calibrate the SEM.
Another aspect of the invention provides for measuring a workpiece feature using the scanning electron microscope to obtain a workpiece feature measurement, and correlating the workpiece feature measurement with at least one calibration factor to obtain a workpiece feature CD. In this way, the calibration factor or factors may be employed to adjust workpiece feature measurements in order to determine or obtain accurate workpiece feature CDs.
According to yet another aspect of the present invention, a system is provided for calibrating an SEM. The system may be utilized to implement the above methods according to the invention. The system may comprise a reference sample having a first line set with generally parallel lines of a first line width and a first pitch, and a second line set with generally parallel lines of a second line width and a second pitch, a scanning electron microscope adapted to measure at least one of the first line width and the first pitch, and at least one of the second line width and the second pitch, and a processor or other device adapted to correlate at least one of the first line width measurement and the first pitch measurement with at least one of the second line width measurement and the second pitch measurement to obtain at least one calibration factor. The system provides for reduction or elimination of calibration errors associated with changing reference sample features such as line width, which were not accounted for in convention SEM calibration.
In accordance with yet another aspect of the invention, there is provided a system for calibrating a scanning electron microscope, comprising a reference sample having a first line set with generally parallel lines of a first line width and a first pitch, and a second line set with generally parallel lines of a second line width and a second pitch, a device for measuring at least one of the first line width and the first pitch, a device for measuring at least one of the second line width and the second pitch, and a device for correlating at least one of the first line width measurement and the first pitch measurement with at least one of the second line width measurement and the second pitch measurement to obtain at least one calibration factor. The calibration system accounts for degradation in a reference sample associated with repeated usage in a SEM, and further allows trending analysis of the reference sample degradation. In this way, a user may identify degradation in a reference sample feature, and take appropriate action without a corresponding degradation in SEM workpiece measurement accuracy.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative examples of the invention. These examples are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.